Energy gain issues for cast on

I have noticed that at times, ball lightning stops generating energy. I can break the bug by casting something else, like spark, and then ball lightning can gain energy again.

It is fairly intermittent, however, it is most common with rare mobs with resistance aura or lightning resistance.

I know the damage reduction can prevent shock from triggering with thresholds, however, the hits do add shock and gain energy, but after a few hits, they stop.